In mass spectrometry (MS), one of the intrinsic features of efficient electrospray or nanoelectrospray processes is the need to add volatile buffer and/or solvent to the sample in order to enable efficient evaporation in a controlled buffering environment. This requirement is sometimes incompatible with pre-spray activities that need to be performed for analytical, separation and/or purification purposes or that are required due to the specific properties of these volatile elements of the spray.
Mixing Sheath Liquid and Sample After a Separation
Different strategies have been presented to overcome this problem, which often consist in adding a pressurised flux conventionally called sheath liquid (often methanol, acetonitrile and acetic or formic acid) at the spraying orifice in order to mix the solution to be sprayed with this sheath liquid. In other systems, a sheath gas (i.e. a pressurised flux of gas, e.g. argon) is used to favour the evaporation of the sample solvent. These configurations, standard for electrospray ionisation (ESI), are compatible with systems that work with imposed and relatively high flow-rates of both the sheath liquid/gas and the solution to be sprayed (normally, larger than 5 microL/min).
In other cases, a liquid junction is introduced by means of a T-cell at the end of the electrospray capillary in order to add about 50% of sheath liquid as make-up flow so as to obtain a good spray. Again, these systems are efficient when the flow rates are large enough and well-controlled, but they often create quite large dead volumes which induce sample dilution and hence affect the sensitivity as well as the resolution of the detection.
In a nanoelectrospray, i.e. when the flow-rate is smaller than 5 microL/min, a liquid junction can also be used, but it is very difficult to control it efficiently because the pressure applied to the sheath liquid to mix with the solution to be sprayed often destabilizes the flow in the main sample capillary. In case of separation, this may deeply reduce the resolution of the separated peaks. Finally, when the system is used for electrophoresis, the pressure applied on the sheath liquid can counter the electroosmotic flow and render the plug profile distorted which decreases the resolution of the separation.
In microanalytical devices, the possibility of fabricating different channels and interconnecting them on the same chip enables one to create liquid junctions with a minimum of dead volume, which reduces the sensitivity and resolution losses. Nevertheless, the main difficulty in electrospray and nanospray sampling with sheath liquids is to control the flow-rate of the sheath liquid and that of the sample solution. These flow-rates of course need to be in the same order of magnitude, so as to enable good and stable spray generation while maintaining a sufficiently high proportion of sample for the detection.
In order to control these flow-rates, some authors have derivatised the surface of a side arm to enable electroosmotic flow in the right direction in both channels, (Ramsey et al., Analytical Chemistry, 1997, vol. 69, 1174). Other groups have integrated a liquid junction in the chip that is connected to a sheath liquid syringe through a capillary (R. D. Smith et al., Electrophoresis, 2000, vol. 21, 191). The microfluidic control in these systems is yet quite difficult and necessitates to fill the different arms of the chip without bubbles before starting the spray with real samples.
Reactions in the Nanoelectrospray
Other applications such as chemical or biological reactions in the nanoelectrospray have been demonstrated and are expected to deliver more information on tiny amounts of samples, particularly in proteomics where some digestions could be performed directly in the spray. For example, nanoelectrosprays with immobilised trypsine have been used to digest a peptide and spray it on-line into the MS, thereby enabling the reaction kinetics to be followed. One of the main drawbacks is that the trypsine, which can work in organic solutions, needs a pH of 8.2 to operate, whereas the spray would be more efficient at a pH of 3. As the volume and the flow rate are too small in the nanoelectrospray, it is difficult to introduce a liquid junction to add the sheath liquid. Therefore, these kinds of direct monitoring of reactions are very limited and are not yet considered as analytical tools.